The purpose of this study is to evaluate the efficacy of humanized monoclonal anti-IL5 antibody, SCH 55700 in reducing peripheral blood eosinophilia in patients with either hypereosinophilic syndrome or eosinophilic gastroenteritis. Patients with hypereosinophilic syndrome refractory to or intolerant of therapy with conventional therapy (steroids, hydroxyurea and interferon alpha) will be admitted on this protocol. Additionally, subjects with eosinophilic gastroenteritis refractory or intolerant to conventional therapy (systemic glucocorticoids) will be admitted on this protocol. A thorough clinical evaluation will be performed with emphasis on potential sequelae of eosinophil-mediated tissue damage. A baseline bone marrow will be obtained to exclude neoplasia and to assess the degree and nature of eosinophilipoiesis. Blood cells and/or serum will also be collected to provide reagents (such as DNA, RNA, and specific antibodies) for use in the laboratory to address issues related to the immunologic basis of FHES and GE as well as their pathogenesis and treatment. Following intravenous administration of a single 1 mg/kg dose of the humanized monoclonal anti-IL5 antibody, SCH 55700, patients will be followed in an inpatient setting for a minimum of 72 hours for monitoring of adverse effects secondary to the infusion. Followup visits will be scheduled monthly for 1 year or until the peripheral eosinophil count returns to baseline. Patients showing a reduction in eosinophilia and evidence of clinical improvement will be eligible to receive 5 additional doses of 1 mg/kg of SCH 55700 at monthly intervals.